Back Side Illumination (BSI) images sensors such as BSI CMOS image sensors are becomingly increasingly popular because of their compatibility with conventional CMOS manufacturing processes, low cost, small size, and high performance. CMOS image sensors are replacing more traditional charge coupled device (CCD) sensors due to certain advantages, including that CMOS image sensors consume less power, are compatible with CMOS semiconductor processes, and the sensors may be integrated with additional CMOS logic devices on a single integrated circuit device. Conventional CMOS manufacturing processes, however, are not tuned for BSI image sensors and current manufacturing processes may cause less than desirable device performance, particularly on the performance criteria of quantum efficiency (QE), signal to noise ratio (SNR), and light mean value uniformity.
In forming a BSI image sensor, semiconductor processing is used to form an array of photodiodes (PDs) and the associated MOS transfer transistors in a sensor array for integrated circuit dies on a semiconductor wafer, and to form periphery circuit MOS devices such as input-output buffers in the integrated circuit dies on the semiconductor wafer. As the photodiodes are exposed to light, an electrical charge is induced in the photodiodes corresponding to the light intensity. The MOS transfer transistor may sample the charge and transfer charge to a storage node for further processing. Color pixels may be formed by placing color filters over the light sensitive CMOS circuitry.
To form the BSI image sensor, after the CMOS photodiodes and MOS transistors are formed, additional semiconductor processing is performed on the back side (the side free from metallization layers and interlayer dielectrics) of the semiconductor wafer. In conventional BSI processing, the processing includes forming a buffer oxide and a metal shield over the wafer. The buffer oxide and metal shield are then selectively removed from the sensor array area of the integrated circuit dies, while the metal shield is retained over the periphery area.
However, in the conventional semiconductor processing, an etch to remove the metal shield from the sensor array areas also removes either a portion of, or all of the buffer oxide layer. The conventional etch process results in either a non-uniform buffer oxide thickness or no oxide thickness over the sensor array areas, which negatively impacts the performance of the BSI image sensor.
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